Reversible crosslinking method for making an electro-optic polymer

ABSTRACT

A crosslinkable second-order nonlinear optical polymer having one or more polarizable chromophore moieties, one or more diene moieties, and one or more dienophile or dienophile precursor moieties, wherein the diene and dienophile moieties are reactive to form 4+2 cycloaddition products; a crosslinked second-order nonlinear optical polymer having aligned, polarizable chromophore moieties and one or more 4+2 cycloaddition moieties, wherein the 4+2 cycloaddition moieties are reversibly, thermally reactive to provide diene moieties and dienophile moieties; lattices and devices that include the crosslinkable second-order nonlinear optical polymer; lattices and devices that include the crosslinked second-order nonlinear optical polymer; and methods for making the crosslinked second-order nonlinear optical polymer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/440,971, filed Jan. 15, 2003.

GOVERNMENT RIGHTS

This invention was made with government support awarded by the Air ForceOffice of Scientific Research (Government Contract No. F49620-01-0364).The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a nonlinear optical polymer useful inelectro-optic devices and a method for making the polymer.

BACKGROUND OF THE INVENTION

Organic second-order nonlinear optical (NLO) polymers have receivedincreasing interests due to their potential for applications inhigh-speed electro-optic (E-O) devices with very broad bandwidth and lowdrive voltage, and that can be made with cost effective fabricationprocess. In order to be qualified for practical devices, a materialneeds to possess simultaneously large and thermally stable E-O activity,and good processibility. Although some of the above-mentionedrequirements have been satisfied individually, the success ofintegrating all these desirable properties in a single material systemhas not yet been realized and remains a very challenging task. Forexample, large E-O coefficients have been demonstrated in severalguest/host poled polymers, but these materials often suffer from lowpoling-induced alignment stability and poor solvent resistance duringthe multi-layer fabrication process. Thus, it is desirable to be able tocovalently incorporate chromophores into a polymer network and hardenthe matrix through crosslinking reactions to improve both thermal andmechanical properties. However, a reduction of 20–40% in E-O activity isusually accompanied with this approach. E-O activity is reduced becausetypical poling of conventional NLO thermoset polymers is achievedthrough a sequential lattice hardening and poling process. As a result,the lattice hardening significantly reduces the chromophoreorientational flexibility due to the increase of glass-transitiontemperature (T_(g)) and interchain entanglements of the polymers, whichseverely inhibit chromophore reorientation under the poling field,resulting in a decreased poling efficiency. In addition, hightemperatures needed for curing these polymers often cause decompositionof highly polarizable chromophores.

To overcome this nonlinearity-stability problem, the lattice hardeningprocess should be ideally separated from the poling process thatrequires high rotation freedom of chromophores. In addition, becausemost of the highly efficient NLO chromophores possess only moderatechemical and thermal stability, very mild conditions should be employedfor lattice hardening.

Accordingly, a need exists for a method for making an NLO polymer thatallows for high rotation freedom of NLO chromophores during the polingprocess and relatively mild conditions for lattice hardening. Thepresent invention seeks to fulfill this need and provides furtherrelated advantages.

SUMMARY OF THE INVENTION

In one aspect of the present invention, second-order nonlinear opticalpolymers are provided. In one embodiment, the nonlinear optical polymersare crosslinkable. The crosslinkable polymers include a chromophoremoiety and a diene and dienophile having reactivity sufficient toprovide a 4+2 cycloaddition product. In another embodiment, thenonlinear optical polymers are crosslinked. The crosslinked polymersinclude a 4+2 cycloaddition product formed by reaction of a diene anddienophile.

In another aspect, the present invention provides a method for makingsecond-order nonlinear optical polymers. In one embodiment of the methodthe steps include poling a crosslinkable polymer having one or morepolarizable chromophore moieties, and one or more diene moieties and oneor more dienophile moieties in an electric field to provide a poledcrosslinkable polymer having aligned, polarizable chromophore moieties;and crosslinking the poled crosslinkable polymer having aligned,polarizable chromophore moieties to provide a crosslinked polymer havingaligned, polarizable chromophore moieties. The polymer crosslinksinclude 4+2 cycloaddition moieties formed by reaction of diene anddienophile moieties. In other aspects of the invention, lattices thatinclude the nonlinear optical polymers and devices that include thenonlinear optical polymers are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of the preparation of arepresentative crosslinkable polymer of the invention (PSDACLD) having asecond-order nonlinear optical chromophore (CLD) moiety, a dienophile(masked maleimide) moiety, and a diene (furan) moiety;

FIG. 2A is a schematic illustration of the irreversible deprotection ofa representative masked dienophile moiety to provide a dienophile(maleimide) moiety useful in the method of the invention;

FIG. 2B is a schematic illustration of the thermally reversiblecrosslinking of a representative dienophile (maleimide) moiety and arepresentative diene (furan) moiety to provide a representative 4+2cycloaddition product useful in making a representative crosslinkednonlinear optical polymer of the invention;

FIG. 3 is a graph illustrating the thermal analysis (10° C./min) of arepresentative nonlinear optical polymer of the invention (PSDACLD);plots 1 (TGA) and 2 (DSC) correspond to polymer samples that were heatedfrom room temperature to 200° C.; plot 3 (DSC) corresponds to a polymersample that was heated from room temperature to 200° C. after heating at125° C. for 30 minutes;

FIG. 4A is a schematic illustration of a representative crosslinkablepolymer of the invention having non-aligned chromophore moieties beforeelectric field poling;

FIG. 4B is a schematic illustration of a representative crosslinkablepolymer of the invention having aligned chromophore moieties afterelectric field poling; and

FIG. 4C is a schematic illustration of a representative crosslinkedpolymer of the invention having aligned chromophore moieties afterlattice hardening through 4+2 cycloaddition of diene and dienophilemoieties.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In one aspect, the present invention provides second-order nonlinearoptical polymers that are useful in electro-optic devices.

In one embodiment, the nonlinear optical polymers are crosslinkable. Thecrosslinkable polymers include one or more polarizable chromophoremoieties, one or more diene moieties, and one or more dienophile ordienophile precursor moieties. The diene and dienophile moieties arereactive to form 4+2 cycloaddition products. In one embodiment, thedienophile moiety is a maleimide moiety. In one embodiment, the dienemoiety is a furan moiety. In one embodiment, the chromophore moietycomprises one or more crosslinkable moieties, such as trifluorovinylether moieties.

In another embodiment, the nonlinear optical polymers are crosslinked.The crosslinked polymers include aligned, polarizable chromophoremoieties, and one or more 4+2 cycloaddition moieties formed by reactionof a diene and dienophile. The 4+2 cycloaddition moieties arereversibly, thermally reactive to provide diene moieties and dienophilemoieties. This feature allows for improved chromophore alignment duringthe poling process.

The crosslinked polymers of the invention are provided by theDiels-Alder [4+2] cycloaddition reaction, which can be carried outduring lattice hardening. The Diels-Alder (DA) reaction involvescovalent coupling of a “diene” with a “dienophile” to provide acyclohexene cycloadduct. See, for example, Kwart, H., and K. King, Chem.Rev. 68:415, 1968. Most DA cycloadditions can be described by asymmetry-allowed concerted mechanism without generating the biradical orzwitterion intermediates. Among many features of the DA reaction is thatthe resultant adducts can be reversibly thermally cleaved to regeneratethe starting materials (i.e., diene and dienophile). For example, theretro-DA reaction has been exploited to thermally crosslink linearpolymers that are capable of reverting to their thermoplastic precursorsby heating. See, for example, (a) Chen, X., et al., Science 295:1698,2002; (b) Gousse, C., et al., Macromolecules 31:314, 1998; (c)McElhanon, J. R., and D. R. Wheeler, Org. Lett. 3:2681, 2001. Thecrosslinked polymers of the invention are prepared by a method thatutilizes process advantages in hardening NLO polymers to achieve apolymer having both high nonlinearity and thermal stability.

As used herein, the term “diene” refers to a 1,3-diene that is reactivetoward a dienophile to provide a 4+2 (Diels-Alder) cycloaddition product(i.e., a cyclohexene). The term “dienophile” refers to an alkene that isreactive toward a diene to provide a 4+2 cycloaddition product. The term“dienophile precursor” refers to a moiety that can be converted to adienophile. Suitable dienes and dienophiles may be unsubstituted orsubstituted.

The crosslinkable and crosslinked polymers of the invention can bethermoplastic polymers. In one embodiment, the crosslinkable polymersare thermoplastic polymers. In one embodiment, the crosslinked polymersare thermoplastic polymers. As used herein, the term “thermoplastic”refers to a polymer or material having the property of softening whenheated and of hardening and becoming rigid again when cooled. Typically,thermoplastic materials can be remelted and cooled time after timewithout undergoing any appreciable chemical change.

As noted above, the crosslinkable polymer includes one or morepolarizable chromophore moieties, one or more diene moieties, and one ormore dienophile or dienophile precursor moieties. The polymer may be anyone of a variety of polymers that includes the chromophore, diene, anddienophile (or dienophile precursor) moieties. Suitable polymers includehomopolymers, copolymers, block copolymers, and grafted polymers. In oneembodiment, the polymer is a homopolymer to which have been grafted thechromophore, diene, and dienophile (or dienophile precursor) moieties.In this embodiment, the polymer (e.g., poly(4-vinylphenol)) has afunctional group (e.g., phenolic hydroxyl) that is suitable for reactionwith suitably functionalized chromophore, diene, and dienophile (ordienophile precursor) compound (e.g., carboxyl group) to covalentlycouple the chromophore, diene, and dienophile (or dienophile precursor)moieties to the polymer backbone (e.g., through an ester link).

The polymers may be prepared through grafting, for example, bycovalently coupling a chromophore moiety, a diene moiety, and adienophile (or dienophile precursor) moiety to a polymer backbone, wherea suitable functional group (e.g., carboxyl group) on the chromophoremoiety, diene moiety, and dienophile (or dienophile precursor) moietyreacts with a suitable functional group on the polymer (e.g., phenolichydroxyl group). Alternatively, the polymer may be prepared by reactinga chromophore containing a polymerizable group, a diene (or dieneprecursor) containing a polymerizable group, and a dienophile (ordienophile precursor) containing a polymerizable group to form apolymer. Combinations of polymerizing and grafting may also be used.

The polymers of the invention include one or more second-order nonlinearoptical chromophore moieties. As used herein, the term “chromophore”refers to a moiety that can absorb a photon of light. Thus, by virtue ofthe presence of one or more chromophore moieties, the polymers of theinvention are chromophores. In the context of the polymers of theinvention, the term “nonlinear” refers second order effects that arisefrom the nature of the polarizable chromophore moieties (i.e.,“push-pull” chromophore moieties) having the general structure D-π-A,where D is an electron donor, A is an electron acceptor, and π is aπ-bridge that conjugates the donor to the acceptor.

A “donor” (represented by “D”) is an atom or group of atoms with lowelectron affinity relative to an acceptor (defined below) such that,when the donor is conjugated to an acceptor through a π-bridge, electrondensity is transferred from the donor to the acceptor.

An “acceptor” (represented by “A”) is an atom or group of atoms withhigh electron affinity relative to a donor such that, when the acceptoris conjugated to a donor through a π-bridge, electron density istransferred from the acceptor to the donor.

A “π-bridge” or “conjugated bridge” (represented in chemical structuresby “π” or it “π^(n)” where n is an integer) is comprised of an atom orgroup of atoms through which electrons can be delocalized from anelectron donor (defined above) to an electron acceptor (defined above)through the orbitals of atoms in the bridge. Preferably, the orbitalswill be p-orbitals on multiply bonded carbon atoms such as those foundin alkenes, alkynes, neutral or charged aromatic rings, and neutral orcharged heteroaromatic ring systems. Additionally, the orbitals can bep-orbitals on multiply bonded atoms such as boron or nitrogen ororganometallic orbitals. The atoms of the bridge that contain theorbitals through which the electrons are delocalized are referred tohere as the “critical atoms.” The number of critical atoms in a bridgecan be a number from 1 to about 30. The critical atoms can also besubstituted further with the following: “alkyl” as defined below, “aryl”as defined below, or “heteroalkyl” as defined below. One or more atoms,with the exception of hydrogen, on alkyl, aryl, or heteroalkylsubstituents of critical atoms in the bridge may be bonded to atoms inother alkyl, aryl, or heteroalkyl substituents to form one or morerings.

Representative chromophores, donors, acceptors, and π-bridges known tothose skilled in the art and useful in making the polymers of theinvention are described in U.S. Pat. Nos. 6,361,717; 6,348,992;6,090,332; 6,067,186; 5,708,178; and 5,290,630; each expresslyincorporated herein by reference in its entirety. Representativechromophores that can be suitably functionalized for coupling to apolymer for making the polymers of the invention are described in WO02/08215; U.S. patent application Ser. No. 10/212,473, filed Aug. 2,2002; U.S. patent application Ser. No. 10/347,117, filed Jan. 15, 2003;and U.S. Provisional Patent Application No. 60/520,802, filed Nov. 17,2003; Adv. Mater. 14(23):1763–1768, 2002; and Adv. Mater.14(19):1339–1365, 2002; each expressly incorporated herein by referencein its entirety.

The polymers of the invention include one or more diene moieties.Suitable diene moieties include any diene (i.e., 1,3-diene) moiety thatis reactive in forming a 4+2 cycloaddition product with a dienophile. Asnoted above, the diene is covalently coupled to the polymer backbone toprovide the polymers of the invention by the reaction of a suitablefunctional group on the diene (e.g., carboxyl group) with a suitablefunctional group on the polymer (i.e., phenolic hydroxyl group). In oneembodiment, the diene moiety includes a furan moiety.

The polymers of the invention include one or more dienophile ordienophile precursor moieties. Suitable dienophile moieties include anydienophile moiety that is reactive in forming a 4+2 cycloadditionproduct with a diene. Suitable dienophile precursor moieties include anydienophile precursor moiety that provides a dienophile that is reactivein forming a 4+2 cycloaddition product with a diene. In one embodiment,the dienophile moiety includes a maleimide moiety. In one embodiment,the dienophile precursor moiety includes a capped maleimide moiety(e.g., furan-capped maleimide).

The synthesis of a representative crosslinkable polymer of the inventionis described in the Example and is illustrated schematically in FIG. 1.FIG. 1 illustrates the preparation of a PSDACLD, apoly(4-vinylphenol)-based polymer that includes a second-order nonlinearoptical chromophore (i.e., CLD) moiety, a dienophile (i.e., maskedmaleimide) moiety, and a diene (i.e., furan) moiety. Although arepresentative polymer is described as having these specific components,it will be appreciated that the polymers of the invention can include avariety of chromophore, dienophiles, and dienes. FIG. 1 also depictspoly(4-vinylphenol) as having n repeating units and the product polymerhaving x units that include the chromophore moiety, y units that includethe dienophile (or dienophile precursor moiety), and z units thatinclude the diene moiety. It will be appreciated that FIG. 1 is aschematic representation of a polymer of the invention and that thepolymer may comprise repeating units that do not include chromophore,diene, or dienophile moieties. It will also be appreciated that thechromophore, diene, and dienophile moieties do not necessarily occur inblocks in the polymer as depicted in FIG. 1.

As depicted schematically in FIG. 1, the crosslinkable polymers of theinvention include three different functional moieties: (1) chromophoremoieties, such as derivatives of CLD-type chromophore, (2) dienophilemoieties, such as capped maleimide, and (3) diene moieties, such asfuran. The functional moieties are covalently attached to a polymerbackbone (e.g., poly(4-vinylphenol)) as side chains to affordcrosslinkable NLO polymer PSDACLD.

In this synthesis, the maleimide (dienophile) is protected with furan toprevent any crosslinking reaction from occurring prior to the latticehardening step. The resultant PSDACLD possesses good solubility incommon organic solvents, such as chloroform and THF. The polymer wascharacterized by ¹H NMR, ¹⁹F NMR, UV-Vis spectroscopy, GPC, and thermalanalysis, as described in the Example. The chromophore loading level inPSDACLD was at about 15 weight percent, confirmed by the relativeintegration comparison of those characteristic peaks in ¹H NMR spectrum.

The furan used for protecting the maleimide moiety can be thermallycleaved by retro-DA reaction and easily evaporated from polymer toprovide the maleimide moiety as dienophile. FIG. 2A is a schematicillustration of the irreversible deprotection of a representative maskeddienophile moiety to provide a dienophile (maleimide) moiety useful inthe method of the invention.

The loss of furan and the formation of the maleimide moiety asdienophile can be clearly verified by thermal analysis. FIG. 3 is agraph illustrating the thermal analysis PSDACLD. The thermal gravimetricanalysis (TGA) (Plot 1) shows a steep weight loss of 4.5 weight percentfrom 110° C. to 150° C., corresponding to an endothermic peak observedin similar temperature range by thermal analysis using differentialscanning calorimeter (DSC) (Plot 2). Isothermal heating of the sample at125° C. for 30 minutes also resumed the weight loss of 4.5 weightpercent, a value that is in good agreement with the content of furanused for protection. After this, the sample became completely insolubleeven after it was rapidly quenched to room temperature, indicating theease of DA crosslinking reaction between the side chains of imido andfuran moieties.

However, this crosslinked network can be dissociated thermally,evidenced by the reappearance of the similar endothermic peak ofretro-DA reaction when the thermally quenched sample was re-heated againby DSC. See FIG. 3, Plot 3. The study from DSC also shows a typicalglass-transition behavior around 100° C. for PSDACLD.

FIG. 2B is a schematic illustration of the thermally reversiblecrosslinking of a representative dienophile (maleimide) moiety and arepresentative diene (furan) moiety to provide a representative 4+2cycloaddition product useful in making a representative crosslinkednonlinear optical polymer of the invention.

In order to improve the poling efficiency, PSDACLD is also covalentlyattached with a fluorinated dendron, which has previously been employedto demonstrate the advantage of the site-isolation effect in side-chaindendronized NLO polymers. See Luo, J. D., et al., Advanced Materials,14:1763, 2002. The highest temperature used in the poling processdescribed above is only about 125° C. Under such conditions, thetrifluorovinyl ether functional groups are not reactive: these groups donot polymerize (no polymerization below 160° C.) and do not interferewith the DA crosslinking reaction by acting as a dienophile. Nocycloaddition products were formed between4-(trifluorovinyloxy)bromobenzene and furan even after refluxingovernight in methanol. The fluorinated dendrons in PSDACLD can be usedto mimic the local environment around the chromophore as for side-chaindendronized NLO polymers and to provide a parallel comparison of thepoling efficiency. See FIG. 1, where the chromophore moiety includes anacceptor moiety substituted with R, a trifluorovinyl ether dendron.

For E-O measurements, a filtered solution (filtered through the 0.2 μmPTFE filter) of PSDACLD in cyclopentanone was spin-coated onto indiumtin oxide (ITO) glass substrates. The films were baked under vacuum at85° C. overnight to ensure removal of the residual solvent, and thenbaked under nitrogen at 125° C. for 30 minutes to evaporate the furanprotecting group. A thin layer of gold was then sputtered onto the filmsas the top electrode for performing the poling experiments. Because thepolymer has been crosslinked upon cooling, the films were again baked at120° C. for 1 hour to revert to its linear thermoplastic polymerprecursor. This is one of many advantages of this approach: retro-DAreaction allows the technique of contact poling even after smallmolecules like furan are evaporated prior to poling. The film was thencooled down to 100° C. and poled at this temperature with a DC electricfield of 1.42 MV/cm. This poling temperature is close to the onsettemperature of the retro-DA reaction (110° C.) and slightly higher thanthe typical temperature range used for DA crosslinking reaction (65 to85° C.). At this temperature, the material possesses the characteristicsof a typical thermoplastic polymer. Concurrently, the chromophores canbe effectively reoriented under the poling field. After the polingprocess, a sequential cooling/curing process (85° C. for 1 hour, 75° C.for 1 hour, and 65° C. for 1 hour) was performed to anneal and crosslinkthe polymer through the DA reaction. All of the above conditions arevery mild and can be tolerated by the chemically and thermally sensitiveCLD-type NLO chromophores in PSDACLD, which was previously precluded byseveral conventional thermoset polymer systems due to the harsh andreactive conditions. The electro-optic (E-O) coefficient (r₃₃) value wasmeasured using the reflection technique at 1.3 μm, and the poled filmafter lattice hardening of PSDACLD showed a very large E-O coefficient(r₃₃=76 pm/V).

Considering the chromophore content is only 15 weight percent inPSDACLD, this value proves that the same high poling efficiency can bereproduced as the side-chain dendronized NLO polymer (97 pm/V and 20weight percent, chromophore content). See Luo, J. D., et al., AdvancedMaterials, 14:1763, 2002. More importantly, the lattice hardening forPSDACLD is separated from its poling process: the poled films can beeffectively hardened at temperatures far below the poling temperature.As a result, the orientation order of chromophores can be maintainedvery well during the crosslinking process, leading to a high E-Oactivity even after the lattice hardening. For comparison, the E-Ocoefficient of a conventional polyurethane thermoset with a similarchromophore has been found to possess a much lower r₃₃ value than thatobtained from the guest-host materials (36 vs. 57 pm/V at 1.06 μm). SeeVan der Boom, M. E., et al., Langmuir 18:3704, 2002.

Due to the efficient lattice hardening of DA reaction, the material alsoexhibited very promising temporal alignment stability: the poled filmsof PSDACLD retains about 80% of its original r₃₃ value after baking at70° C. for several hundred hours.

A representative crosslinked polymer of the invention is illustratedschematically in FIG. 4C. The crosslinked polymer includes thechromophore moieties as described above for the crosslinkable polymer.In the crosslinked polymer, at least some of the diene and dienophilegroups present in the crosslinkable polymer have been reacted to provide4+2 cycloaddition products. Referring to FIG. 4C, crosslinked polymer 30includes aligned, polarizable chromophore moieties 32 (arrows depictchromophore dipole alignment), with optional dendrons 34, grafted topolymer backbone 31, which is crosslinked through 4+2 cycloadditionmoieties 36.

In another aspect, the present invention provides a method for making acrosslinked polymer having electro-optic activity. Generally, acrosslinkable polymer of the invention is heated to provide a softenedpolymer, which is poled to align its chromophore moieties, and thenhardened (i.e., crosslinked) by cooling to provide a crosslinked polymerhaving aligned chromophore moieties. As used herein, the term “softened”refers to a polymer or material of the invention in which itschromophore moieties are sufficiently mobile to align in an electricfield. The term “hardened” refers to a polymer or material of theinvention in which its chromophore moieties are restricted in mobilityand cannot readily align or depole.

In one embodiment, the method includes the steps of heating acrosslinkable polymer of the invention to form a softened polymer;subjecting the softened polymer to an electric field to provide a poledpolymer having aligned, polarizable chromophore moieties; and coolingthe poled polymer to a temperature sufficient to provide a hardened,crosslinked polymer having electro-optic activity.

In this embodiment of the method, a crosslinkable polymer having one ormore polarizable chromophore moieties, and one or more diene moietiesand one or more dienophile moieties is poled in an electric field toprovide a poled crosslinkable polymer having aligned, polarizablechromophore moieties. The poled crosslinkable polymer having aligned,polarizable chromophore moieties is then crosslinked to provide acrosslinked polymer having aligned, polarizable chromophore moieties.The polymer crosslinks include 4+2 cycloaddition moieties formed byreaction of diene and dienophile moieties.

A representative method for making the crosslinked polymer of theinvention is illustrated schematically in FIGS. 4A–4C. FIG. 4A shows arepresentative crosslinkable polymer of the invention having non-alignedchromophore moieties before electric field poling. The crosslinkablepolymer 10 includes polarizable chromophore moieties 12 (arrows depictchromophore dipole alignment) with optional dendrons 14, dienophilemoieties 16, and diene moieties 18 grafted to polymer backbone 11.Crosslinkable polymer 20, which may be a separate polymer that mayinclude chromophore moieties or another portion of crosslinkable polymer10, also includes dienophile moieties 26 and diene moieties 28. FIG. 4Bschematically illustrates crosslinkable polymer 10 after electric fieldpoling (e.g., at about 100° C.). Referring to FIG. 4B, crosslinkablepolymer 10 includes aligned, polarizable chromophore moieties 12 (arrowsdepict chromophore dipole alignment) with optional dendrons 14,dienophile moieties 16, and diene moieties 18. FIG. 4C schematicallyillustrates a representative crosslinked polymer of the invention havingaligned chromophore moieties after lattice hardening (65–85° C.) through4+2 cycloaddition of diene and dienophile moieties on neighboringpolymers (or the same polymer at remote sites). Referring to FIG. 4C,crosslinked polymer 30 includes aligned, polarizable chromophoremoieties 32 (arrows depict chromophore dipole alignment) with optionaldendrons 34, grafted to polymer backbone 31, which is crosslinkedthrough 4+2 cycloaddition moieties 36.

In one embodiment, the method further includes the steps of heating thehardened, crosslinked polymer at a temperature sufficient to provide asoftened, crosslinkable polymer; subjecting the softened, crosslinkablepolymer to an electric field to provide a poled crosslinkable polymer;and cooling the poled crosslinkable polymer to a temperature sufficientto provide a hardened, crosslinked polymer having electro-opticactivity.

In this embodiment, the initially formed crosslinked polymer is heatedat a temperature sufficient to cause one or more of the 4+2cycloaddition moieties to react (retro-DA) to form one or more dienemoieties and one or more dienophile moieties to provide a crosslinkablepolymer. The crosslinkable polymer is then poled to provide a poledpolymer having an increased number of aligned chromophore moieties. Thepoled polymer having an increased number of aligned chromophore moietiesis then crosslinked to provide a second crosslinked, poled polymerhaving increased aligned chromophore moieties compared to the initiallyformed crosslinked polymer. These steps may be repeated to furtherenhance chromophore alignment.

In other aspects of the invention, materials (e.g., lattices) thatinclude the nonlinear optical polymers and devices that include thenonlinear optical polymers are provided.

The materials and methods described herein can be useful in a variety ofelectro-optic applications. In addition, these materials and methods maybe applied to polymer transistors or other active or passive electronicdevices, as well as OLED (organic light emitting diode) or LCD (liquidcrystal display) applications.

The use of organic polymers in integrated optics and opticalcommunication systems containing optical fibers and routers has beenpreviously described. The compounds, molecular components, polymers, andcompositions (hereinafter, “materials”) may be used in place ofcurrently used materials, such as lithium niobate, in most type ofintegrated optics devices, optical computing applications, and opticalcommunication systems. For instance, the materials may be fabricatedinto switches, modulators, waveguides, or other electro-optical devices.

For example, in optical communication systems devices fabricated fromthe polymers of the invention may be incorporated into routers foroptical communication systems or waveguides for optical communicationsystems or for optical switching or computing applications. Because thematerials are generally less demanding than currently used materials,devices made from such polymers may be more highly integrated, asdescribed in U.S. Pat. No. 6,049,641, which is incorporated herein byreference. Additionally, such materials may be used in periodicallypoled applications as well as certain displays, as described in U.S.Pat. No. 5,911,018, which is incorporated herein by reference.

Techniques to prepare components of optical communication systems fromoptically transmissive materials have been previously described, and maybe utilized to prepare such components from materials provided by thepresent invention. Many articles and patents describe suitabletechniques, and reference other articles and patents that describesuitable techniques, where the following articles and patents areexemplary:

Eldada, L. and L. Shacklette, “Advances in Polymer Integrated Optics,”IEEE Journal of Selected Topics in Quantum Electronics 6(1):54–68,January/February 2000; Wooten, E. L., et al. “A Review of LithiumNiobate Modulators for Fiber-Optic Communication Systems,” IEEE Journalof Selected Topics in Quantum Electronics 6 (1):69–82, January/February2000; Heismann, F., et al. “Lithium Niobate Integrated Optics: SelectedContemporary Devices and System Applications,” Optical FiberTelecommunications III B, Academic, Kaminow and Koch (eds.), New York,1997, pp. 377–462; Murphy, E., “Photonic Switching,” Optical FiberTelecommunications III B, Academic, Kaminow and Koch (eds.), New York,1997, pp. 463–501; E. Murphy, Integrated Optical Circuits andComponents. Design and Applications, Marcel Dekker, New York, August1999; Dalton, L., et al., “Polymeric Electro-Optic Modulators: FromChromophore Design to Integration with Semiconductor Very Large ScaleIntegration Electronics and Silica Fiber Optics,” Ind. Eng. Chem. Res.38:8–33, 1999; Dalton, L., et al., “From Molecules to Opto-Chips:Organic Electro-Optic Materials,” J. Mater. Chem. 9:1905–1920, 1999;Liakatas, I. et al., “Importance of Intermolecular Interactions in theNonlinear Optical Properties of Poled Polymers,” Applied Physics Letters76(11): 1368–1370, Mar. 13, 2000; Cai. C., et al.,“Donor-Acceptor-Substituted Phenylethenyl Bithiophenes: Highly Efficientand Stable Nonlinear Optical Chromophores,” Organic Letters1(11):1847–1849, 1999; Razna, J., et al., “NLO Properties of PolymericLangmuir-Blodgett Films of Sulfonamide-Substituted Azobenzenes,” J. ofMaterials Chemistry 9:1693–1698, 1999; Van den Broeck, K., et al.,“Synthesis and Nonlinear Optical Properties of High Glass TransitionPolyimides,” Macromol. Chem. Phys 200:2629–2635, 1999; Jiang, H., and A.K. Kakkar, “Functionalized Siloxane-Linked Polymers for Second-OrderNonlinear Optics,” Macromolecules 31:2508, 1998; Jen, A. K.-Y.,“High-Performance Polyquinolines with Pendent High-TemperatureChromophores for Second-Order Nonlinear Optics,” Chem. Mater.10:471–473, 1998; “Nonlinear Optics of Organic Molecules and Polymers,”Hari Singh Nalwa and Seizo Miyata (eds.), CRC Press, 1997; Cheng Zhang,Ph.D. Dissertation, University of Southern California, 1999; GalinaTodorova, Ph.D. Dissertation, University of Southern California, 2000;U.S. Pat. Nos. 5,272,218; 5,276,745; 5,286,872; 5,288,816; 5,290,485;5,290,630; 5,290,824; 5,291,574; 5,298,588; 5,310,918; 5,312,565;5,322,986; 5,326,661; 5,334,333; 5,338,481; 5,352,566; 5,354,511;5,359,072; 5,360,582; 5,371,173; 5,371,817; 5,374,734; 5,381,507;5,383,050; 5,384,378; 5,384,883; 5,387,629; 5,395,556; 5,397,508;5,397,642; 5,399,664; 5,403,936; 5,405,926; 5,406,406; 5,408,009;5,410,630; 5,414,791; 5,418,871; 5,420,172; 5,443,895; 5,434,699;5,442,089; 5,443,758; 5,445,854; 5,447,662; 5,460,907; 5,465,310;5,466,397; 5,467,421; 5,483,005; 5,484,550; 5,484,821; 5,500,156;5,501,821; 5,507,974; 5,514,799; 5,514,807; 5,517,350; 5,520,968;5,521,277; 5,526,450; 5,532,320; 5,534,201; 5,534,613; 5,535,048;5,536,866; 5,547,705; 5,547,763; 5,557,699; 5,561,733; 5,578,251;5,588,083; 5,594,075; 5,604,038; 5,604,292; 5,605,726; 5,612,387;5,622,654; 5,633,337; 5,637,717; 5,649,045; 5,663,308; 5,670,090;5,670,091; 5,670,603; 5,676,884; 5,679,763; 5,688,906; 5,693,744;5,707,544; 5,714,304; 5,718,845; 5,726,317; 5,729,641; 5,736,592;5,738,806; 5,741,442; 5,745,613; 5,746,949; 5,759,447; 5,764,820;5,770,121; 5,76,374; 5,776,375; 5,777,089; 5,783,306; 5,783,649;5,800,733; 5,804,101; 5,807,974; 5,811,507; 5,830,988; 5,831,259;5,834,100; 5,834,575; 5,837,783; 5,844,052; 5,847,032; 5,851,424;5,851,427; 5,856,384; 5,861,976; 5,862,276; 5,872,882; 5,881,083;5,882,785; 5,883,259; 5,889,131; 5,892,857; 5,901,259; 5,903,330;5,908,916; 5,930,017; 5,930,412; 5,935,491; 5,937,115; 5,937,341;5,940,417; 5,943,154; 5,943,464; 5,948,322; 5,948,915; 5,949,943;5,953,469; 5,959,159; 5,959,756; 5,962,658; 5,963,683; 5,966,233;5,970,185; 5,970,186; 5,982,958; 5,982,961; 5,985,084; 5,987,202;5,993,700; 6,001,958; 6,005,058; 6,005,707; 6,013,748; 6,017,470;6,020,457; 6,022,671; 6,025,453; 6,026,205; 6,033,773; 6,033,774;6,037,105; 6,041,157; 6,045,888; 6,047,095; 6,048,928; 6,051,722;6,061,481; 6,061,487; 6,067,186; 6,072,920; 6,081,632; 6,081,634;6,081,794; 6,086,794; 6,090,322; and 6,091,879.

The foregoing references provide instruction and guidance to fabricatewaveguides from materials generally of the types described herein usingapproaches such as direct photolithography, reactive ion etching,excimer laser ablation, molding, conventional mask photolithography,ablative laser writing, or embossing (e.g., soft embossing). Theforegoing references also disclose electron donors and electron bridgesthat may be incorporated into the polymers of the invention or that mayalso incorporate an electron donor and/or electron bridges describedherein.

Components of optical communication systems that may be fabricated, inwhole or part, with materials according to the present inventioninclude, without limitation, straight waveguides, bends, single-modesplitters, couplers (including directional couplers, MMI couplers, starcouplers), routers, filters (including wavelength filters), switches,modulators (optical and electro-optical, e.g., birefringent modulator,the Mach-Zender interferometer, and directional and evanescent coupler),arrays (including long, high-density waveguide arrays), opticalinterconnects, optochips, single-mode DWDM components, and gratings. Thematerials described herein may be used with, for example, wafer-levelprocessing, as applied in, for example, vertical cavity surface emittinglaser (VCSEL) and CMOS technologies.

In many applications, the materials described herein may be used in lieuof lithium niobate, gallium arsenide, and other inorganic materials thatcurrently find use as light-transmissive materials in opticalcommunication systems.

The materials described herein may be used in telecommunication, datacommunication, signal processing, information processing, and radarsystem devices and thus may be used in communication methods relying, atleast in part, on the optical transmission of information. Thus, amethod according to the present invention may include communicating bytransmitting information with light, where the light is transmitted atleast in part through a material including a polymer of the invention orrelated macrostructure.

The materials of the present invention can be incorporated into variouselectro-optical devices. Accordingly, in another aspect, the inventionprovides electro-optic devices including the following:

an electro-optical device comprising a polymer or related macrostructureaccording to the present invention;

a waveguide comprising a polymer or related macrostructure according tothe present invention;

an optical switch comprising a polymer or related macrostructureaccording to the present invention;

an optical modulator comprising a polymer or related macrostructureaccording to the present invention;

an optical coupler comprising a polymer or related macrostructureaccording to the present invention;

an optical router comprising a polymer or related macrostructureaccording to the present invention;

a communications system comprising a polymer or related macrostructureaccording to the present invention;

a method of data transmission comprising transmitting light through orvia a polymer or related macrostructure according to the presentinvention;

a method of telecommunication comprising transmitting light through orvia a polymer or related macrostructure according to the presentinvention;

a method of transmitting light comprising directing light through or viaa polymer or related macrostructure according to the present invention;

a method of routing light through an optical system comprisingtransmitting light through or via a polymer or related macrostructureaccording to the present invention;

an interferometric optical modulator or switch, comprising: (1) an inputwaveguide; (2) an output waveguide; (3) a first leg having a first endand a second end, the first leg being coupled to the input waveguide atthe first end and to the output waveguide at the second end; and 4) anda second leg having a first end and a second end, the second leg beingcoupled to the input waveguide at the first end and to the outputwaveguide at the second end, wherein at least one of the first andsecond legs includes a polymer or related macrostructure according tothe present invention;

an optical modulator or switch, comprising: (1) an input; (2) an output;(3) a first waveguide extending between the input and output; and (4) asecond waveguide aligned to the first waveguide and positioned forevanescent coupling to the first waveguide; wherein at least one of thefirst and second legs includes a polymer or related macrostructureaccording to the present invention, the modulator or switch may furtherincluding an electrode positioned to produce an electric field acrossthe first or second waveguide; and

an optical router comprising a plurality of switches, wherein eachswitch includes: (1) an input; (2) an output; (3) a first waveguideextending between the input and output; and (4) a second waveguidealigned to the first waveguide and positioned for evanescent coupling tothe first waveguide; wherein at least one of the first and second legsincludes a polymer or related macrostructure according to the presentinvention, the plurality of switches may optionally be arranged in anarray of rows and columns.

The present invention provides a facile and reversibly thermallycrosslinkable nonlinear optical (NLO) polymer system for overcoming thenonlinearity-stability tradeoff in poled polymers. The inventionprovides high poling efficiency via effective site-isolation inside-chain dendronzied NLO polymers that is maintained after an isolatedand mild process of lattice hardening via the Diels-Alder crosslinkingreaction. The resulting material exhibits a combination of very largeelectro-optic coefficient (r₃₃ value of 76 pm/V at 1.3 μm) and excellenttemporal stability at 70° C.

The following example is provided for the purpose of illustrating, notlimiting, the invention.

EXAMPLE Synthesis and Characterization of a RepresentativeThermo-Reversibly Crosslinkable NLO Polymer: PSDACLD

General methods. Dichloromethane was distilled over phosphorus pentoxideunder nitrogen. Tetrahydrofuran (THF) was distilled from sodiumbenzophenone ketyl under nitrogen prior to use.4-(Dimethylamino)-pyridinium 4-toluenesulfonate (DPTS) was preparedaccording to J. S. Moore, S. I. Stupp, Macromolecules 23:70, 1990.Compounds 2, 3, and 4 were synthesized by following the similarprocedures described in Luo, J. D., et al., Advanced Materials 14:1763,2002, and Yoon, S. S., and W. C. Still, Tetrahedron 51:567, 1995. Allthe other chemicals were purchased from Aldrich unless otherwisespecified.

PSDACLD Synthesis. The synthesis of PSDACLD is illustrated schematicallyin FIG. 1.

To a solution of poly(4-vinylphenol) (1, 0.045 g, 0.374 mmol), compound2 (0.120 g, 0.094 mmol), and DPTS (0.027 g, 0.092 mmol) in the mixtureof 7.5 mL of THF and 3 mL of dichloromethane was added 0.024 g ofdicyclohexylcarbodiimide (DCC) (0.12 mmol). The reaction mixture wasallowed to stir at room temperature for 12 h under nitrogen, and 0.055 gof compound 3 (0.15 mmol) and 0.038 g of DCC (0.18 mmol) were added, andstirred at room temperature for another 12 h. Then, 0.043 g of compound4 (0.17 mmol) and 0.045 g of DCC (0.22 mmol) were added, and thereaction mixture was stirred at room temperature for 24 h. Iterativedissolving and filtration removed most of the resultant urea. Thefiltered dichloromethane solution was then added dropwise to stirringmethanol. The precipitate was collected and reprecipitation by addingdropwise its dichloromethane solution into methanol afforded polymerPSDACLD as blue-greenish solid (0.170 g, yield 75%). ¹H NMR (CDCl₃, TMS,ppm): δ 7.85–8.3, 7.46 (d), 7.07–7.42 (br m), 7.05 (d, J=8.3 Hz),6.06–7.26 (br m), 4.82–5.42 (br m), 5.02 (br s), 4.08–4.66 (br m),3.59–3.92 (br m), 3.08–3.51 (br m), 2.72 (br s), 2.08–2.50 (br m),0.59–2.10 (br m). ¹⁹F NMR (CDCl₃, C₆F₆, ppm): −135.35 (dd, J=54.9 Hz),−127.7 (dd, J=97.6 Hz), −120.9 (dd, J=61 Hz). UV-Vis spectrum in1,4-dioxane: λ_(max)=662 nm. T_(g)=100° C.

PSDACLD Thermal Analysis. Thermal analysis was performed by thermalgravimetric analysis (TGA) differential scanning calorimeter (DSC) withthe heating rate of 10° C./min under nitrogen. The results were shown inFIG. 3, which is a graph illustrating the thermal analysis PSDACLD.Plots 1 (TGA) and 2 (DSC) correspond to polymer samples that were heatedfrom room temperature to 200° C. In Plot 1, the arrow depicts a 4.5%weight loss corresponding to loss of furan, deprotection of themaleimide to provide the dienophile. In Plot 2, the arrow depicts anendothermic peak corresponding to loss of furan, deprotection of themaleimide to provide the dienophile. Plot 3 (DSC) corresponds to apolymer sample that was heated from room temperature to 200° C. afterheating at 125° C. for 30 minutes. In Plot 3, the arrow depicts anendothermic peak corresponding to de-crosslinking (retro-Diels Alder)generating diene and dienophile moieties.

PSDACLD Film Preparation and Electro-Optic (E-O) Measurement. For E-Omeasurement, the solution of PSDACLD in cyclopentanone (12 wt %,filtered through a 0.2 μm syringe filter) was spin-coated onto theindium tin oxide (ITO) glass substrates. The film was baked under vacuumat 85° C. overnight to ensure the removal of the residual solvent. Ther₃₃ value was measured using the reflection technique at 1.3 μm asdescribed in Teng, C. C., and H. T. Man, Appl. Phys. Lett. 56:1734, 1990

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A crosslinkable polymer, comprising (a) one or more polarizablechromophore moieties; (b) one or more diene moieties; and (c) one ormore dienophile or dienophile precursor moieties; wherein the diene anddienophile moieties are reactive to form 4+2 cycloaddition products. 2.The polymer of claim 1, wherein the dienophile moiety comprises amaleimide moiety.
 3. The polymer of claim 1, wherein the diene moietycomprises a furan moiety.
 4. The polymer of claim 1, wherein thechromophore moiety comprises one or more crosslinkable moieties.
 5. Thepolymer of claim 4, wherein the crosslinkable moieties comprisetrifluorovinyl ether moieties.
 6. The polymer of claim 1, wherein thepolymer is thermoplastic.
 7. A crosslinked polymer, comprising (a)aligned, polarizable chromophore moieties; and (b) one or more 4+2cycloaddition moieties, wherein the 4+2 cycloaddition moieties arereversibly, thermally reactive to provide a polymer having dienemoieties and dienophile moieties.
 8. The polymer of claim 7, wherein thedienophile moiety comprises a maleimide moiety.
 9. The polymer of claim7, wherein the diene moiety comprises a furan moiety.
 10. The polymer ofclaim 7, wherein the polymer is thermoplastic.
 11. A lattice, comprisingthe crosslinkable polymer of claim
 1. 12. A lattice, comprising thecrosslinked polymer of claim
 7. 13. An electro-optic device, comprisingthe crosslinkable polymer of claim
 1. 14. An electro-optic device,comprising the crosslinked polymer of claim 7.